Conventionally, as a bearing that supports an axle for a railway car, a double row cylindrical bearing and a back-to-back (referred to as “outward” hereinafter) double row tapered roller bearing are used in many cases.
Since high load is applied to the axle for the railway car during running, while the axle rotates, bending in a direction perpendicular to a rotation axis is generated. At this time, small sliding (referred to as “fretting” hereinafter) is generated between the axle and the inner ring of the bearing supporting the axle, which causes the problem that the inner peripheral surface end of the inner ring interferes with the axle and a scar is generated in the outer peripheral surface of the axle (referred to as “axle scar” hereinafter). Conventionally, in order to avoid the above problem, a railway car axle supporting structure in which measures shown in FIG. 1 are taken, for example has been known. A double row tapered roller bearing 1 supporting an axle 10 for a railway car comprises an inner ring 2 having flange parts at both ends, an outer ring 3, tapered rollers 4 arranged between the inner ring 2 and the outer ring 3 in double rows, a retainer 5 retaining the intervals of the tapered rollers 4, and an oil seal case 6 and an oil seal 7 for sealing both ends thereof.
A groove 10a is formed at a part corresponding to an inner peripheral surface end 2a of the inner ring 2, in the outer peripheral surface of the axle 10. Thus, the inner peripheral surface end 2a of the inner ring 2 can be prevented from interfering with the axle when the bending is generated. This can be applied to a double row cylindrical roller bearing.
Here, since the double row tapered roller bearing 1 is the outward bearing in which the small diameter side ends of the tapered rollers 4 abut on each other, the inner ring 2 is a split inner ring in which two inner ring members abut on each other. In another case, a filler piece is further provided between the two inner ring members.
FIG. 2 is an enlarged view showing one of the split inner ring at an abutment part S in FIG. 1. As shown in FIG. 2, the inner diameter surface of the inner ring 2 comprises a contact part 2b fitted to the axle 10 at its center part, a non-contact part 2c apart from the axle in a diameter direction at its outer edge part, and a ridge line part 2d on the boundary between the contact part 2b and the non-contact part 2c. The ridge line part 2d interferes with the axle 10 when the bending is generated in the axle 10 like the inner peripheral surface end of the inner ring 2. As a result, the axle scar could be generated in the outer peripheral surface of the axle 10 at the abutment part S of the inner ring 2.
Thus, in order to prevent the axle scar from being generated at the abutment part S, a method of reducing the surface pressure of the ridge line part 2d by cutting a shoulder part 2e of the abutment surface of the inner ring 2 has been used as disclosed in Japanese Unexamined Patent Publication No. 2004-84938.
However, it is difficult to provide a groove in the outer peripheral surface of the axle 10 at the part corresponding to the abutment part S in order to prevent the interference between the inner ring 2 and the axle 10 at the abutment part S in addition to the groove 10a at the part corresponding to the bearing end 2a in view of the processing cost of the axle 10.
Therefore, in order to prevent the scar from being generated at the abutment part S, means for preventing the generation of the scar is to be provided on the side of the double row tapered roller bearing 1 like the method disclosed in the Japanese Unexamined Patent Publication No. 2004-84938. However, according to the method disclosed in the above document, the surface pressure reducing effect of the ridge line part 2d is small, so that the generation of the scar of the axle 10 is not satisfactorily prevented.
In addition, according to another example of the bearing supporting the axle for the railway car, as shown in FIG. 3, a double row tapered roller bearing 101 supporting an axle 110 for the railway car comprises an inner ring 102 in which small diameter side ends of two inner ring members abut on each other, an outer ring 103, tapered rollers 104 arranged between the inner ring 102 and the outer ring 103 in double rows, a retainer 105 retaining the intervals of the tapered rollers 104, and a seal 106 sealing both ends thereof.
FIG. 4 is an enlarged view showing one of the inner ring member at an abutment part P in FIG. 3. As shown in FIG. 4, the inner diameter surface of the inner ring 102 has a fit part 102a fitted to the axle 110 at its center part, an insertion guide part 102b at its outer edge part, and a ridge line part 102c on the boundary between the fit part 102a and the insertion guide part 102b. 
In a case where bending is generated in the axle 110, for example, as shown in FIG. 3, when the upper side of the axle 110 is bent in the shape of convex and receives tensile stress and the lower side thereof is bent in the shape of concave and receives compressive stress, fretting is generated between the axle 110 and the ridge line part 102c at the part P, which could cause the abrasion and the scar in an outer peripheral surface 110a of the axle 110. Thus, stress concentration occurs at the axle scar, which could cause a serious accident such as the breakdown of the axle.
Thus, in order to prevent the generation of the axle scar at the abutment part P, the method of reducing the surface pressure of the ridge line part 102c by cutting a shoulder part 102d of the abutment surface of the inner ring 102 has been known as disclosed in the Japanese Unexamined Patent Publication No. 2004-84938.
FIG. 5 is an enlarged view showing the vicinity of a part Q of the double row tapered roller bearing 101 shown in FIG. 3. As shown in FIG. 5, since the inner ring 102 is in contact with the whole region of the end face of a rear lid 107, displacement of the double row tapered roller bearing 101 is limited by the rear lid 107.
Thus, the double row tapered roller bearing 101 cannot follow the displacement of the axle 110 and the contact surface pressure of the ridge line part 102c is increased when the axle 110 is bent in a radial direction. In addition, the surface pressure reducing effect is not provided sufficiently only by thinning the shoulder part 102d of the abutment surface of the inner ring 102. Therefore, it means that there still exists the problem that the abrasion and the axle scar caused by the fretting between the ridge line part 102c and the axle 110 could be generated. In addition, this problem could occur at the contact part between the inner ring 102 and an oil thrower 108.